Martensitic stainless steel sheets have an excellent degree of corrosion resistance, rigidity, and wear resistance, and thus, may be used for manufacturing various types of metallic instruments and industrial instruments, and, in particular, for razor blades, scalpels for medical use, general knives and scissors, and the like.
In general, martensitic stainless steel may be manufactured by producing slabs using ingot steel subjected to a continuous casting process or using ingot steel while reheating and hot rolling the slabs so as to produce hot rolled strips, performing a batch annealing process to anneal the hot rolled strips, and then performing a pickling process in order to remove scale formed thereon during the hot rolling process. In addition, after the pickling process, a cold rolling process of a material or a process for the processing of a produced product may be applied.
In a structure of steel formed after hot rolling, among the manufacturing processes, a martensite phase, a tempered martensite phase, a ferrite phase, a residual austenite phase, and the like are mixed, and after hot rolling annealing, the structure of steel is transformed to ferrite and carbide to be nitrocarburized. Such a nitrocarburized material (a soft material) undergoes a final heat treatment process so as to be transformed into martensitic steel.
On the other hand, in the case that metallic instruments have a higher level of desired quality, a relatively high degree of rigidity is required. Such a degree of rigidity may be achieved through a basic martensitic structure of steel. Such a martensitic structure is a significantly light fine structure generated at the time of rapidly cooling high-temperature austenite, and as the content of carbon soluble in high temperature austenite phases is increased, carbon soluble in martensite is increased, such that the rigidity of martensite may be increased. Accordingly, in order to manufacture martensitic stainless steel having a relatively high degree of rigidity, the content of carbon therein should be able to be increased.
However, as the content of carbon in steel is increased, a degree of segregation becomes severe and high area coexistence is increased so as to be significantly vulnerable to casting. Thus, martensitic stainless steel has mainly been manufactured through an ingot casting method. However, in the case of the ingot casting method, coarse precipitates and central segregation may occur at grain boundaries, due to a slow cooling rate, thereby causing a deterioration of quality in post processing.
Recently, as a method replacing the ingot casting method, a method of manufacturing martensitic stainless steel using a twin roll strip caster has emerged, so that the occurrence of central segregation may be suppressed and the precipitation of chrome carbon may be reduced at an initial grain boundary to improve a level of quality.
However, even in a case in which martensitic stainless steel is cast using such a twin roll strip caster, since fine structures generated while being pressed and coagulated in a casting roll of a strip caster may also remain in finally produced products, uniformity may be degraded.